This invention relates to a monochrome or color printing method and an apparatus for realizing the same and in particular to a scanning recording type printing method and an apparatus for improving the image quality in a high precision fine image recording apparatus.
As a method for varying the recording area of each pixel in order to express light and shade of the image in a scanning recording type printing apparatus, there is known a method, by which image recording pulse signals are modulated in pulse width by means of data representing the tone. Apparatuses described in Japanese Patent Application un-examined publications (Kokais) Nos. 82-57679 and 82-99866 are examples of this type of system.
In such a printing recording apparatus it is necessary to reduce each cell in size and increase the pixel density in order to be able to record an image with high precision and high fineness. The scanning direction and the size of each pixel in the scanning recording are determined by the scanning speed and the production period of the image recording pulse signal. Consequently, in order to make each pixel smaller, the production period of the image recording pulse signal must be shortened and the rate of the intermission must be increased. However, when the rate of the intermission of the image recording pulse signal is increased, the image quality has a tendency to be lowered.
The reason will be explained as follows taking an electro-graphic laser beam printer as an example.
In FIG. 2, a memory device 1 stores tone data of each of the pixels in image signals coming from an image read-out device or a computer (not shown in the figure) for one scanning line. The tone data are sent to a latch 2 in the form of pixel tone data DA for every pixel, depending on the position of recording scanning by a pixel clock signal PCLK1 given by a timing treatment circuit 4, which will be described later. Supposing that the pixel tone is represented by 16 degrees from "0" (white) to "15" (black), the pixel tone data DA are 4 bit data. In a pixel recording pulse signal generation circuit 9 the latch 2 holds (latches) the pixel tone data DA by a pixel clock signal PCLK2 given by the timing treatment circuit 4 and its holding period of time is equal to a period of time during which one pixel domain is scanned for recording. These pixel tone data DA held by the latch 2 are given to a comparator 5. A counter 3 which is a cyclic 4 bit binary counter, counts clock signals CLK1 coming from a clock generator 10 under the control by a recording scanning signal LINE1 from the timing treatment circuit 4. 16 clock signals CLK1 are outputted for a period of time during which one pixel domain is scanned for recording. The counter 3 counts up from "0" (white) to "15" (black) and gives the content of the count as comparison data DB to the comparator 5. At the same time it gives a carry signal as pixel clock signal PCLK3 to the timing treatment circuit 4. The timing treatment circuit 4 generates the pixel clock signals PCLK1 and PCLK2, referring to the pixel clock signal PCLK3 and at the same time uses a detection signal LINE2 coming from a laser beam detector 8 as a recording scanning start synchronization signal for every scanning line.
The comparator 5 compares the pixel tone data DA with the comparison data DB and generates a 2-value pixel recording pulse signal S, corresponding to EQU "black", if DA&gt;DB EQU "white", if DA.ltoreq.DB,
which is given to a semiconductor laser circuit 6. A laser beam outputted by the semiconductor laser circuit 6 is deflected in a region of an angle .theta. so as to scan and illuminate an electro-graphic photo-sensitive drum 7. In this way an electro-static latent image is formed thereon and transferred to a recording paper, after having being developed with toner. After that, it is further fixed so as to be a record.
FIGS. 1(A) to 1(C) indicate a timing chart representing the working mode of the pixel recording pulse signal and the pixel recording in such a laser beam printer. (A) indicates the pixel number and the pixel tone data DA. The abscissa t in (B) represents the time, in which T denotes the period of time necessary for scanning to record one pixel. The ordinate represents digital values corresponding to pixel tones, in which "0" indicates "white"; "15" indicates "black"; DA shows the pixel tone data; and DB shows the comparison data. The abscissa x in (C) represents the position of the recording scanning of the laser beam and the hatched regions show the recorded area for each of the pixels.
In such a recording method, since the laser beam outputted by the semiconductor laser circuit 6 has a certain spread in the scanning direction, when this laser beam is interrupted by the pixel recording pulse signal S in the course of the scanning, the light quantity at both the border portions of the recorded dots in the main scanning direction on the pixel recording surface is inconveniently in an intermediate region between white and black and thus the tone of the record at these portions is unstable, which is a factor lowering the image quality. This is produced by the fact that the laser beam has a certain spread. Consequently, when, in order to record a finer image with a high precision, pixels are made smaller and the number of interruptions of the laser beam is increased, the proportion of such unstable regions increases, which decreases the image quality.
Such phenomena are not limited to the laser beam printer, but is produced in photo-sensitive recording apparatuses, in which recording energy given to a recording medium is interrupted and controlled in the course of scanning, stylus electro-static recording apparatuses, and scanning recording type recording apparatuses such as scanning illumination type electrographic printers using liquid crystal light switches and light emitting diodes.
In addition, in color printing by off-set printing, it is difficult to position dots to be printed with a high precision. For example, in the case of a multi-color printing with 4 colors of cyan, yellow, magenta and India ink, when it is tried to superpose corresponding dots of different colors on each other, slight misalignment produces Moire fringes (interference fringes). Therefore, in practice, the screen angles of dots of different colors are intentionally varied appreciably so that the dots of different colors are superposed at random, in order to prevent the production of low frequency moire fringes. However, by this method, superposition of dots of different colors is irregular, which prevents the effect of theoretical color correction.
In contrast, in a digital printer such as a laser beam printer, etc., since it is possible to position dots fairly precisely when it is tried to superpose corresponding dots of different colors on each other, there are produced no Moire fringes.
An article by SAYANAGI published in Denshi-Shashin Gakkaishi (Journal of the Electro-Graphic Society) 23, No. 3 (1984) (in Japanese) has disclosed a "concentric solution model", by which the dots are printed by a digital printer so that their centers are superposed on each other (cf. FIG. 3(A)) and reported that 100% under color removal (UCR) is possible by this method (cf. FIG. 3(B)). If this concentric solution model could be realized ideally, a perfect UCR (100% UCR) and other various color correction theories would be efficacious. However, this concentric solution model has not taken the following points into consideration.
(1) Although the dots formed by printing are, in general, ideally printed at the central portion, they are not precisely printed at the peripheral portion because of scattering of ink or unevenness of printing. According to the concentric solution model, since the net points other than the dot of the ink, which is at the top, exhibit their color by their peripheral portion, it is difficult to reproduce the precise color.
(2) When the UCR is effected according to the concentric solution model, since a color dot by an Indian ink color is at the top, other inks printed under the black dot come to nothing and in addition, the dot is apt to be transferred inperfectly because of the superposition of useless inks.
(3) Even by a digital printer, the net points of different colors deviate more or less from each other because of expansion or contraction of the paper, etc. The concentric solution model is poor at this position divergence and the risk that Moire fringes are produced is high.